Eukaryotic organisms have evolved a "mitotic checkpoint" to monitor the metaphase-anaphase transition in order to assure high fidelity transmission of genetic information to daughter cells. This checkpoint prevents progression through mitosis prior to attachment of every centromere to microtubules of the mitotic spindle. In attempting to identify how checkpoint signaling is linked to microtubule attachment, earlier work from the Cleveland lab has proposed that the centromere- bound microtubule motor CENP-E is one link which provides microtubule attachment, relaying this to the centromere-bound checkpoint kinase BUBR1. The focus of this proposal is to test this model for CENP-E involvement in linking the vertebrate checkpoint to microtubule attachment, focusing on the ability to manipulate the cell cycle in vitro (using Xenopus egg extracts).